1. Field Of The Invention
The invention relates to reversible storage means for media, with a storage core which with heating releases the stored medium or on which the medium is stored with release of heat.
2. Prior Art
Hydrogen is a particularly "clean" fuel (combustion product: water; no carbon dioxide) and could therefore be widely used in future. To be able to use it to drive vehicles, however, the storage problem must be solved first. This is where metal hydrides for instance come in, which represent a very safe storage medium for hydrogen. What is known as low-temperature hydrides (LTH) so far have been used for this purpose almost exclusively. LTH have the drawback, however, that only a relatively small amount of hydrogen can be stored per kilogram of metal.
High-temperature hydrides (HTH) have about three times the storage capacity of LTH, but require relatively large amounts of heat at high temperatures to release the hydrogen. The exhaust heat from combustion-engine-driven vehicles is insufficient for this, and vehicles with electric (fuel cell) drive produce practically no exhaust heat at all. Consequently, the experiments with HTH have not been very promising so far.
Several embodiments of such reversible storages are known, and reference is made particularly to metal hydrides. But a drawback of the known storages is that they require outside energy for starting. But outside energy is not always available.
For example, "Development Trends of Hydrogen-Driven Vehicles with Hydride Storage", Haus der technik, lecture prepublication, pp. 40-46, describes a rechargeable hydride storage arrangement in which a high-temperature hydride storage is coupled to a low-temperature hydride storage. This reference notes, however, (page 41, left column, middle) that "to withdraw hydrogen from the hydride, heat must always be supplied."
The necessity for supplying an outside energy source is disadvantageous, however. For example, in the case where hydrogen is used to drive a motor vehicle, the outside energy is usually sought from the exhaust gas heat. But this energy source is not available when the vehicle is being started. Therefore, another energy source is required to start the engine.
The device is a combination of an HTH storage cell, an LTH storage cell and a catalyst. It is started by inflowing air which triggers catalytic combustion of part of the hydrogen (e.g., from the low-temperature hydride storage); the combustion heat is sufficient to discharge the HTH cell, so that no external heat is needed. Another advantage is that flameless catalytic combustion involves only little pollutant emission and is therefore considerably "cleaner" than additional heating with a flame-producing burner. Catalytic combustion takes place at temperatures below 500.degree. C., which means that nitrogen oxides (NO.sub.x) are avoided. If appropriate catalysts are used, hydrogen may even react at 0.degree. C. Catalytic combustion is highly efficient (up to 99.9% fuel utilization). Thermodynamic efficiency is particularly great in all applications requiring low- or medium-temperature heat. Heat losses can be minimized by relatively simple measures.
The device can also be designed as a heat supply unit; in this case the stored hydrogen is used up completely to generate heat. Also, the device makes it possible to fully utilize the excellent safety properties of this storage medium.